Two-phase refrigerant distribution system for parallel tube evaporator coils

ABSTRACT

A parallel tube heat exchanger receiving two-phase refrigerant flow from a common manifold is provided with an eductor nozzle in one end of the manifold so as to cause the flow of refrigerant to pass along one leg of the manifold and down another leg thereof so as to reenter the upstream end of the first leg by way of a crossover tube. In this way, the two-phase refrigerant flow to the parallel tubes is more uniformly distributed. In one embodiment of the invention, the manifold is a unitary tube with a centrally disposed, longitudinally extending partition to define two longitudinally extending chambers that fluidly communicate by gaps at the ends of the central partition.

BACKGROUND OF THE INVENTION

This invention relates generally to heat exchangers and particularly tothe distribution of two-phase, gas-liquid flows among the severalparallel circuits of multitube heat exchangers used as refrigerantevaporators. More particularly, the invention relates to this type ofmultitube evaporator wherein the distribution of gas and liquid flowsare made to the multiple parallel circuits from a common manifold or“header”.

It is desirable that this distribution of the two-phase refrigerant flowamong the circuits be uniform with regards to both the total mass flowsand the relative mass flows of gas and liquid. Unequal total flows andespecially unequal flows of liquid among the parallel circuits canresult in incomplete evaporation in some circuits and inefficient use ofthe surface in others.

The difficulty associated with the achieving the desired refrigerantdistribution lies with the nature of two-phase gas-liquid flow whereinthe two phases tend to separate spatially and chronologically on a macroscale. Good distribution is especially difficult from low flow orstagnating regions of a manifold that are remote from the refrigerantentry. The tendency for flow separation is exacerbated if thedistributing manifold has a vertical component to its orientationwherein gravitational forces contribute further to flow separation.

Evaporators with very few parallel circuits can be fed successfully bymethods that avoid the use of the manifold header. A common establishedapproach uses a high velocity jet impingement on a target area thatcauses a local homogeneous flow region from which individual feedertubes lead to the first tube of each circuit. This approach is expensivefrom both the material content and the complexity of assembly,especially as the number of circuits increases. Another common approachdiscards the conventional expansion device and substitutes individualcapillary tubes or orifices for each circuit. This latter approach issimilarly expensive and becomes inapplicable to evaporators with verylarge numbers of circuits.

The usual approach to achieving the desired distribution from a manifoldheader is to cause the refrigerant in the distributor to flow in ahomogenous micro scale mode, that is, one having very short variationsin space and time. This type of two-phase flow is associated with highvelocities. The invention described herein uses this approach. It isespecially appropriate for the flat tube multiport constructionpioneered by the automotive industry for condensing duty, wherein whenapplied as a refrigerant evaporator there are an extraordinary number ofparallel circuits to be fed.

SUMMARY OF THE INVENTION

The unique characteristic of the distribution system that constitutesthe invention is a manifold/header that facilitates a continuouslycirculating two phase flow having a sufficiently homogenous character tofeed the parallel circuits with the desired uniformity of mass flows andrelative mass flows of gas and liquid. Motive force for the circulatingtwo-phase flow is provided by a high velocity jet flow of refrigerantthrough a nozzle from the high pressure side of the refrigerant system.Low fluid static pressure in the region of the high velocity jet inducesthe recirculation of refrigerant from what would otherwise be thestagnating downstream end of the manifold through a return passage.

A generic embodiment of the invention is a linear manifold/header systemcomprised of two separate conduits arranged parallel to one another andconnected fluidly at both ends. The two-phase refrigerant is fed at veryhigh velocity through a nozzle into one of the conduits and is directeddown the length thereof, passing the entrances of the multiple parallelcircuits and feeding them in the desired uniform fashion. Return passageof the remaining flow to the nozzle region is facilitated by the secondconduit and the fluidly connected ends. In later descriptions, thelinear two conduit manifold/header system can be referred to as being atwo pass device. That is, there are two distinct conduits connectedfluidly at their ends, the first carrying the flow in one direction andthe other returning the flow to complete the circulating loop.Variations in the locations of the nozzle and circuit entrances aroundthe circulating loop are possible.

Actual embodiments of the invention will fall into two main categories.In the first category, each pass of the two-pass system is comprised ofa separate conduit. In the second category, the two passes are a singleconstruction.

Briefly, in accordance with one aspect of the invention, a two-passmanifold is provided for conducting the flow of two-phase refrigerantfirst along a first pass and then back along a second pass, with the endof the second pass fluidly communicating with the entrance to the firstpass. The parallel tubes are fluidly interconnected to either of thefirst or second passes and are generally orthogonally orientatedrelative thereto. A draft tube eductor is installed in the first passand is provided a source of two-phase refrigerant flow such that a flowof refrigerant from the eductor not only pumps refrigerant along thefirst pass but also draws the flow of refrigerant flow from the end ofthe second pass to the beginning of the first pass.

By another aspect of the invention, the two-pass manifold comprises asingle conduit with an internal, longitudinally extending partition,with a partition terminating short of each end of the manifold. Therefrigerant flow is then caused to flow into the manifold, on one sideof the partition, pass between the end of the partition and the end ofthe manifold into the other side of the partition, and pass back alongthe other side of the manifold to reenter the first side at the otherend of the partition.

In the drawings as hereinafter described, there are two embodimentsdepicted; however, various other modifications and alternateconstructions can be made thereto without departing from the true spiritand scope of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic illustration of one embodiment of the presentinvention.

FIG. 2 is a schematic illustration of an alternate embodiment of thepresent invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring now to FIG. 1 there is shown a heat exchanger with avertically orientated manifold 11 that comprises a first pass tube 12and a second pass tube 13 interconnected at their ends by a return bend14. A plurality of parallel tubes 16 are fluidly connected to and extendorthogonally from the second pass tube 13 as shown or, alternatively,they could extend orthogonally from the first pass tube 12. The end ofthe second pass 13 is fluidly interconnected to an entrance to the firstpass 12 by way of a connector pipe 17.

If the flow of refrigerant were introduced into the first pass 12without further modification of the design so as to flow around thereturn bend 14 and down the second pass tube 13, there would likely be amal-distribution of two-phase refrigerant to various parallel tubes 16.That is, those near the top of the second pass tube can be starved forrefrigerant flow, whereas those closer to the bottom of the second passtube 13 are more likely to be flooded with the liquid refrigerant.

To reduce the likelihood of this occurrence, a draft tube eductor 18,having an inlet end 19 and a discharge end 21 is provided at the inletend of the first pass tube 12. Operation of the heat exchanger withinthe circuit operates as follows. The refrigerant passes from acompressor 22 to a condenser 23 and an expansion device 24 beforepassing into the inlet 19 of the draft tube eductor. The motive fluid isthen caused to flow into the first pass tube 12 as shown by the arrows,around the return bend 14, down the second pass tube 13 and across theconnector pipe 17 to reenter the first pass tube 12. In this way, theflow of two-phase refrigerant to the parallel tubes 16 is more uniformlydistributed because of the improved circulation caused by the draft tubeeductor 18.

Referring now to FIG. 2, there is shown a plurality of parallel flattubes 26 orientated vertically and fluidly interconnected to a manifold27 orientated horizontally. For simplicity, the tubes are shown as asmall number of tubes, but in actuality, each of these tubes hasmultiple ports such that the refrigerant flow passes to a relativelylarge number of ports from the manifold 27. The manifold 27 is a unitarystructure having end walls 28 and 29 with upper longitudinal wall 31 andlower longitudinal wall 32. A longitudinally extending partition 33 isdisposed within the manifold 27, with the partition 33 having ends 34and 36 which do not extend all the way to the end walls 28 and 29 butwhich, with those end walls, define respective end gaps 37 and 38. Thepartition 33 separates a lower chamber 41 from an upper chamber 42. Adraft tube eductor 39 extends into the lower chamber 41 defined by thepartition 33 and the lower longitudinal wall 32, and is supplied withthe source of two-phase refrigerant as described hereinabove. Operationof the heat exchanger then occurs as follows.

Two-phase refrigerant from the expansion valve is fed through an inletof the draft tube eductor 39 to the lower chamber 41 of the manifold 27,thereby inducing additional flow of two-phase refrigerant from the upperchamber 42 and through the end gap 37. The disturbed flow regionassociated with the draft tube eductor 39 is contained below thepartition 33 and does not unduly affect the flow distribution among theflat tube inlets that are connected to the upper chamber 42. As therefrigerant flow proceeds along the lower chamber 41, the two-phase flowstabilizes by the time it reaches the end gap 38 passing into the upperchamber 42. Thus, as the refrigerant flow passes into the upper chamber42 it is stabilized when it feeds the individual flat tubes 26 as ittravels in a counterclockwise direction. The stabilized two-phaserefrigerant flow continues at a diminishing rate as it flows to the leftend of the upper chamber 42. The end gap 37 allows the diminishedtwo-phase flow of refrigerant to be drawn downwardly into the lowerchamber 41 where it is induced into the high velocity flow coming out ofthe inlet of the draft tube eductor 39 to be recirculated.

It should be mentioned that in order for the draft tube eductor 39 tooperate in such a manner as to draw the flow downwardly from the upperchamber 42, its discharge stream, and preferably the tube itself, mustproject to a point downstream of the end gap 37. Further, it should berecognized that the stabilization of the refrigerant flow does notrequire the full length of the manifold 27. Accordingly, it is possibleto have the tube eductor 39 extend substantially along the length of thelower chamber 41. Alternatively, rather then extending into one endthereof as shown, the draft tube eductor 39 could be inserted into thelower longitudinal wall 32 and so orientated that its discharge flowwould be directed so as to cause counterclockwise flow within themanifold.

While the present invention has been particularly shown and describedwith reference to preferred and alternate embodiments as illustrated inthe drawings, it will be understood by one skilled in the art thatvarious changes in detail may be effected therein without departing fromthe true spirit and scope of the invention as defined in the claims.

1. A heat exchanger having a plurality of parallel tubes for receivingtwo-phase refrigerant flow from a common manifold, comprising: aplurality of tubes aligned in substantially parallel relationship forsimultaneously conducting the flow of two-phase refrigerant downstreamfrom inlet ends thereof; a manifold having first and second passes forserially conducting the flow of two-phase refrigerant along the lengthof said first pass and then along the length of said second pass to anentrance of said first pass; said plurality of tubes being fluidlyinterconnected at their entrance ends to said second pass for receivingtwo-phase refrigerant flow therefrom; and an inducer nozzle fluidlyconnected with said first pass, said nozzle having inlet and dischargeends and being connected by a source of two-phase refrigerant at itsinlet end for providing motive flow of two-phase refrigerant into saidfirst pass.
 2. A heat exchanger as set forth in claim 1 wherein saidfirst and second passes are substantially parallel to each other.
 3. Aheat exchanger as set forth in claim 1 wherein said first and secondpasses are fluidly connected by a return bend.
 4. A heat exchanger asset forth in claim 1 wherein said tubes are substantially orthogonal tosaid manifold.
 5. A heat exchanger as set forth in claim 1 wherein saidinducer nozzle is disposed at one end of said first pass.
 6. A heatexchanger as set forth in claim 5 wherein said inducer nozzle isco-axially orientated with respect to said first pass.
 7. A heatexchanger as set forth in claim 1 wherein said manifold is a unitarystructure with a longitudinally extending divider disposed therein todefine upper and lower chambers.
 8. A heat exchanger as set forth inclaim 7 wherein said divider does not extend the full length of themanifold but leaves gaps at each end thereof between the ends of thedivider and the respective ends of the manifold.